1. Field of the Invention
The present invention relates to a liquid crystalline compound, a liquid crystal composition and a liquid crystal display element. More specifically, the invention relates to a chroman compound, a liquid crystal composition including the same and having a nematic phase, and a liquid crystal display element including the composition.
2. Related Art
On a liquid crystal display element, classification based on an operating mode of liquid crystals includes PC (phase change), TN (twisted nematic), STN (super twisted nematic), BTN (Bistable twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment) and so forth. Classification based on drive systems of elements is PM (passive matrix) and AM (active matrix). The PM (passive matrix) is classified into static, multiplex and the like. The AM is classified into TFT (thin film transistor), MIM (metal insulator metal) and so forth.
These elements contain a composition having suitable physical properties. In order to improve the properties of elements, it is preferable that the composition has suitable physical properties. General physical properties necessary for a compound as an ingredient of the composition are as follows:    (1) chemical stability and physical stability;    (2) a high clearing point. The clearing point is the transition temperature of a liquid crystalline phase—an isotropic phase;    (3) a low lower limit temperature of a liquid crystalline phase. The liquid crystalline phase means a nematic phase, smectic phase and the like;    (4) a small viscosity;    (5) a suitable optical anisotropy;    (6) a suitable dielectric anisotropy. A composition having a large dielectric anisotropy often has a large viscosity; and    (7) a large specific resistance.
A composition is prepared by mixing many compounds. Accordingly, a compound is preferably sufficiently miscible with other compounds. The element may sometimes be used at a temperature below the freezing point, therefore compounds having good compatibility at low temperatures are preferable. A compound having a high clearing point or a low lower limit temperature of a liquid crystalline phase contributes to a wide temperature range of a nematic phase of the composition. A preferable composition has a small viscosity and an optical anisotropy suitable for the mode of the element. A large dielectric anisotropy of a compound contributes to a low threshold voltage of the composition. Such composition serves for obtaining an element having such properties as usability in a wide range of temperature, a short response time, a large contrast ratio, a low drive voltage, a low power consumption and a large voltage retention.
Conventionally, as a chroman compound, following compound (15) is reported for example (refer to JP H6-256339 A). However, compound (15) does not have a sufficiently large dielectric anisotropy.

Conventional techniques are described in such documents as JP H6-256337 A/1994; JP H5-025158 A/1993; JP 2001-316347/2001; JP H6-256339 A/1994; Molecular Crystals and Liquid Crystals (1996), 290, 49-65; Journal of Organic Chemistry (1994), 59(20), 5930-5936; Ferroelectrics (1993), 148(1-4), 195-202 and Helvetica Chimica Acta (1985), 68(5), 1406-1426. However, a more preferable liquid crystalline compound, liquid crystal composition and liquid crystal display element are desired.